Abstract: The definition of entangled state is that it cannot be expressed in a product state，however，in some quantum systems，the entangled states which are used to describe them could be approximately equal to the product states. Conceptual discussion for the approximate equations of such type can be used to explain that this is a source of producing coherent superposition of states of single particle，and it is very possibly the only source.

Key words: entangled state, product state, coherent superposition of states of single particle